1. Field of the Invention
The present invention relates to an optical head device.
2. Description of the Related Art
An optical memory technology using optical disks having pit patterns as high-density and large-capacity recording media has increasing applications such as digital audio disks, video disks, document file disks and data files. According to the optical memory technology, information is recorded onto and reproduced from optical disks with high precision and reliability through minutely converged light beams.
The precision and stability of the recording and reproduction depend entirely on the optical system.
Basic functions of the optical head device being the main part of the optical system are broadly divided into convergence to form diffraction limited minute spots, focus control and tracking control of the optical system, and detection of pit signals. These functions are realized by combinations of various types of optical systems and photoelectric conversion detection methods according to the purposes and uses thereof.
On the other hand, in recent years, high-density and large-capacity optical disks called DVDs have been put to practical use and spotlighted as information media capable of handling a large amount of information such as moving images. In DVD optical disks, in order to increase the recording density, the pit size on the information recording surface is small compared to compact disks (hereinafter, abbreviated as CDs) being conventional optical disks. Therefore, in the optical head devices for performing recording and reproduction of DVD optical disks, the wavelength of the light source for deciding the spot diameter and the numerical aperture (hereinafter, abbreviated as NA) of the converging lens are different from those in the case of CDs. In the case of CDs, the wavelength of the light source is substantially 0.78 .mu.m and the NA is substantially 0.45, whereas in the case of DVD optical disks, the wavelength of the light source is substantially 0.63 to 0.65 .mu.m and the NA is substantially 0.6. Therefore, to perform recording and reproduction of two kinds of optical disks of CDs and DVD optical disks by use of one optical disk drive, an optical head device having two optical systems is necessary.
On the other hand, in view of demands for smaller, thinner and lower-cost optical head devices, the trend is to use a common optical system for CDs and DVDs where possible. For example, a method is used in which a light source for DVDs is used as the light source and only as the converging lens, two kinds of converging lenses one of which is for DVD optical disks and the other of which is for CDs are used, or in which the converging lens is also shared and only the NA is mechanically or optically changed so as to be large for DVD optical disks and small for CDs.
Of the above-described optical head devices, the method to optically change the NA of the converging lens will hereinafter be described with reference to the drawings. In the x, y and z coordinates shown in the lower left part of the figures, the same coordinate axes represent the same directions on the figures.
FIG. 8 shows the structure of the optical system of the conventional optical head device. In FIG. 8, reference numeral 1 represents a semiconductor laser with a wavelength of substantially 0.65 .mu.m. The semiconductor laser 1 is disposed so as to emit a light beam polarized in the direction of the x-axis of the x, y and z coordinates shown in the lower left part of FIG. 8. Reference numeral 2 represents the light beam emitted from the semiconductor laser 1. Reference numeral 3 represents a collimator lens that converts the light beam 2 into a parallel light beam. Reference numeral 4 represents a polarization anisotropic hologram disposed so as to transmit a polarized light beam having its plane of polarization within the x-z plane of FIG. 8 and diffract a polarized light beam having its plane of polarization within the y-z plane. The hologram pattern of the polarization anisotropic hologram 4 is formed so that the direction of diffraction is different between the central part and the peripheral part and that the diffracted light beam from the central part is converted into a plurality of light beams having different focus positions. Reference numeral 5 represents a quarter-wave plate that converts a linearly polarized light beam into a circularly polarized light beam. Reference numeral 6 represents an objective lens. The NA of the objective lens 6 is 0.6. Reference numeral 7 represents an optical disk. Reference numeral 9 represents a first diffracted light beam which is a light beam diffracted at the central part of the polarization anisotropic hologram 4. Reference numeral 8 represents a second diffracted light beam which is the other light beam diffracted by the polarization anisotropic hologram 4. The position of convergence of the first diffracted light beam 9 is closer to the collimator lens 3 than that of the second diffracted light beam 8. Reference numeral 10 represents a photodetector comprising a plurality of photodetection areas.
The operation of the optical head device structured as described above will hereinafter be described.
In FIG. 8, first, the light beam 2 emitted from the semiconductor laser 1 is a linearly polarized light beam having its plane of polarization within the x-z plane of the x, y and z coordinates shown in the lower left part of the figure. After converted into a parallel light beam by the collimator lens 3, the light beam 2 is incident on the polarization anisotropic hologram 4. Since the polarization anisotropic hologram 4 transmits a polarized light beam having its plane of polarization within the x-z plane and diffracts a polarized light beam having its plane of polarization within the y-z plane, the light beam 2 is transmitted by the polarization anisotropic hologram 4 as it is and is then converted into a circularly polarized light beam by the quarter-wave plate 5. The circularly polarized light beam is converged by the objective lens 6 to form a minute spot on the information recording surface of the optical disk 7.
However, since the thickness from the substrate surface to the information recording surface is different between CDs and DVD optical disks, although a minute spot with hardly any aberration can be formed when the optical disk 7 is a DVD optical disk, when the optical disk 7 is a CD, a spot sufficient for reproduction of the CD cannot be obtained because of aberration generation.
It is known that for reproduction of CDs, by using only light, of within approximately 0.38 in terms of the NA, of the light passing through the objective lens 6, the aberration generation is reduced and an excellent spot is obtained.
The light reflected at the information recording surface of the optical disk 7 passes through the objective lens 6 and the quarter-wave plate 5 to be converted into a linearly polarized light beam having its plane of polarization within the y-z plane, and is diffracted by the polarization anisotropic hologram 4.
In the polarization anisotropic hologram, the direction of diffraction is different between the area of the central part through which light, of within approximately 0.38 in terms of the NA of the objective lens 6, of the reflected light beam passes, and the area of the peripheral part. The light from the area of the central part becomes the diffracted light beams 9 and 8, and is converged by the collimator lens 3. At this time, the position of convergence is different between the diffracted light beam 9 and the diffracted light beam 8. The diffracted light beam 9 is converged at a position closer to the collimator lens 3 than the diffracted light beam 8.
The diffracted light beams 9 and 8 are incident on the photodetector 10 to be detected. By computing the output of the photodetector 10, a focus error signal is obtained. The focus error signal is obtained by the above-described method irrespective of whether the optical disk 7 is a DVD optical disk or a CD.
For an information signal, the detection method is different according to whether the optical disk 7 is a DVD optical disk or a CD.
That is, in the case of DVD optical disks, the information signal is obtained from all the diffracted light beams diffracted at the area of the central part and the area of the peripheral part of the polarization anisotropic hologram 4.
On the contrary, in the case of CDs, the information signal is obtained from the diffracted light beams 9 and 8 diffracted at the central part of the polarization anisotropic hologram 4.
As described above, in the case of CDs, by using, of the light reflected from the information recording surface, a light beam, with little aberration, of within approximately 0.38 in terms of the NA of the objective lens 6 for detection of the information signal, excellent signal detection is enabled.
Examples of the method of detecting the focus error signal include a known spot size detection (SSD) method disclosed in Japanese Laid-open Patent Application No. Hei 2-185722. This method will be detailed by use of FIGS. 9(a) to 10(c).
FIGS. 9(a) to 9(c) are views of assistance in explaining the method of detecting the focus error signal. In the figures, the same elements as those of FIG. 8 are denoted by the same reference numerals, and descriptions thereof are omitted.
In FIG. 9(a), the position of the information recording surface of the optical disk 7 is on the side of the focus position of the objective lens 6 which side is farther from the objective lens 6. The focus positions of the diffracted light beam 9 and the diffracted light beam 8 are closer to the collimator lens 3.
In FIG. 9 (b) , the position of the information recording surface of the optical disk 7 coincides with the focus position of the objective lens 6. The focus positions of the diffracted light beam 9 and the diffracted light beam 8 are symmetrical with respect to the surface of the photodetector 10, so that the beam sizes of the diffracted light beams 9 and 8 on the photodetector 10 are the same.
In FIG. 9(c), the position of the information recording surface of the optical disk 7 is on the side of the focus position of the objective lens 6 which side is closer to the objective lens 6. The focus positions of the diffracted light beam 9 and the diffracted light beam 8 are farther from the collimator lens 3.
The spots of the diffracted light beams on the photodetector 10 in the conditions of FIGS. 9(a), 9(b) and 9(c) are shown in FIGS. 10(a), 10(b) and 10(c), respectively.
In FIGS. 10(a) to 10(c), reference numeral 10 represents the photodetector, and reference numerals 8a to 8d and 9a to 9d represent divisional light beams of the diffracted light beam 9 and the diffracted light beam 8 in FIGS. 9(a) to 9(c). This division is performed by dividing the hologram element 4 into areas. Reference numerals 11 to 14 represent some of the detection areas of the photodetector 10. The sum of the outputs of the detection area 11 and the detection area 14 is represented by FE1, and the sum of the outputs of the detection area 12 and the detection area 13 is represented by FE2.
FIG. 10(a) is a view corresponding to FIG. 9(a). In FIG. 10(a) , the sizes of the diffracted light beam spots 8a to 8d are smaller than those of the diffracted light beam spots 9a to 9d.
Moreover, FIG. 10(b) is a view corresponding to FIG. 9(b). In FIG. 10(b), the sizes of the diffracted light beam spots 8a to 8d are the same as those of the diffracted light beam spots 9a to 9d.
Moreover, FIG. 10(c) is a view corresponding to FIG. 9(c). In FIG. 10(c), the sizes of the diffracted light beam spots 8a to 8d are larger than those of the diffracted light beam spots 9a to 9d.
Here, the quadrants of the divisional diffracted light beam spots 8a to 8d and 9a to 9d are shown in FIGS. 11(a) to 11(c) being virtually joined for ease of explanation.
FIG. 11(a) is a view corresponding to FIG. 10(a). Reference numerals 15 and 16 represent diffracted light beam spots formed by joining the diffracted light beam spots 8a to 8d and 9a and 9d shown in FIG. 10(a), respectively.
Reference numerals 17 to 22 represent detection areas. The detection area 17 corresponds to the detection area 11 shown in FIG. 10(a) . The detection area 18 corresponds to the detection areas 12 and 13. The detection area 19 corresponds to the detection area 14. The detection area 20 corresponds to the detection area 13. The detection area 21 corresponds to the detection areas 11 and 14. The detection area 22 corresponds to the detection area 12.
The focus error signal is obtained by the difference between the outputs FE1 and FE2 of the detection areas (FE1-FE2). The focus error signal is negative in the case of FIG. 11(a), is 0 in the case of FIG. 11(b) and is positive in the case of FIG. 11(c).
Therefore, by moving the objective lens 6 so that the focus error signal is 0, the focus position of the objective lens 6 and the position of the information recording surface of the optical disk 7 can be made to coincide with each other.
In the above-described conventional structure, however, when recording and reproduction of CD optical disks are performed, the position where the focus error signal is 0 and the position where the jitter (the value of time-axis variation) of the information signal is minimum do not always coincide with each other.
Hereinafter, this problem will be described with reference to the drawings.
FIGS. 12(a) and 12(b) are schematic views of convergence conditions of the objective lens.
FIG. 12(a) shows a manner of convergence on a DVD optical disk. Reference numeral 23 represents a DVD optical disk substrate. Reference numeral 24 represents the information recording surface. In DVD optical disks, the distance from the substrate surface to the information recording surface is 0.6 mm. Reference numeral 25 represents beams of the converged light.
FIG. 12(b) shows a manner of convergence on a CD optical disk. Reference numeral 26 represents a CD optical disk substrate.
Reference numeral 27 represents the information recording surface. In CD optical disks, the distance from the substrate surface to the information recording surface is 1.2 mm. Reference numeral 28 represents beams of the converged light.
As shown in FIG. 12(a) , in the case of DVD optical disks, the beams 25 of the converged light are incident on the DVD optical disk substrate 23 and are converged on the information recording surface 24 without any aberration.
However, when light is converged on a CD optical disk by the same objective lens, since the distance from the substrate surface to the information recording surface is different from that in the case of DVD optical disks, spherical aberration is generated, so that as shown in FIG. 12(b) , the farther a beam is from the optical axis, the farther the position of convergence of the beam is from the objective lens. Consequently, the beams 28 of the converged light cannot be converged to one point.
As a result of an optical analysis, the inventors have found that in the above-described case, the jitter of the information signal is minimum when the information recording surface is situated substantially at the position of the average focal length, that is, when the wavefront aberration of the converged light beam spot on the information recording surface is substantially minimum. With respect to the sizes of the detection areas 18 and 21 of the photodetector 10 used for the conventional focus error signal detection by the above-described SSD method, the widths thereof are the same, and further, are set so as to be larger than the minimum diameter that the diffracted light beam spots 15 and 16 can have as shown in FIGS. 11(a) to 11(c). These are set on the precondition that the density of the reflected light is uniform like in the case of DVD optical disks.
Therefore, under such a condition, in the case of DVD optical disks, the focus error signal is 0 when the spot size (see FIG. 9(b) of the light converged by the objective lens 6 is minimum on the information recording surface, and the jitter is also minimum at this time. However, in the case of CD optical disks, since the density of the reflected light is nonuniform because of spherical aberration, the jitter of the information signal is not minimum at the position where the focus error signal is 0. That is, the focus error signal is 0 at a position different from the position of the average focal length.
That is, in the conventional optical head device, when a DVD optical disk is used, since the light converged by the objective lens is converged to one point, the position where the jitter of the information signal is minimum can be detected irrespective of the sizes of the detection areas of the photodetector 10. However, when a CD optical disk is used, since the position where the focus error signal by the SSD method is 0 and the position where the jitter of the information signal is minimum are different because of spherical aberration, the time-axis variation of the information signal cannot be reduced.
Moreover, in the above-described conventional structure, in the case of recording and reproduction of DVD optical disks, when the base material thickness of a DVD varies, the difference (focus offset) between the position where the jitter (the value of time-axis variation) of the information signal is minimum and the position where the focus signal is 0 varies.
That is, when the base material thickness of a DVD varies, a large difference in spherical aberration variation is caused between the light beam using all the NAs to obtain a DVD reproduction signal and the light beam being incident on the photodetector for focus detection where the NA is limited to obtain a CD reproduction signal, so that the focus offset significantly varies.